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                堡垒                </h1>
                <p>时间限制：10s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：128MB</p>			</div>
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                                <h3>题目描述</h3><p><div style="margin: 0cm 0cm 0pt 0.5pt; text-indent: -0.5pt"><span style="font-size: 12pt">有个国家，它的所有城市如下图被公路连成了一个环，依次编号为</span><span style="font-size: 12pt">1..N</span><span style="font-size: 12pt">。并且在这个环内有一些笔直的公路，每条公路连接两个城市。为了不发生交通事故，任意两条公路不会在中途相交。为了使城市之间的交通更便利，城市间会建尽可能多的公路，但两个城市之间最多建一条公路。</span></div>
<div style="margin: 0cm 0cm 0pt 0.5pt; text-indent: -0.5pt"></div>
<div style="margin: 0cm 0cm 0pt 0.5pt; text-indent: -0.5pt"><span style="font-size: 12pt">&nbsp;&nbsp;&nbsp; </span><span style="font-size: 12pt">现在因为有另一个国家想破坏这个国家的交通系统，所以我们需要在一些城市建堡垒，使每条公路都能被监督（在</span><span style="font-size: 12pt">i</span><span style="font-size: 12pt">城市建堡垒可以监督与</span><span style="font-size: 12pt">i</span><span style="font-size: 12pt">城市相邻的公路）。问你最少需要建多少个堡垒。</span></div>
<p><img height="314" width="343" alt="" src="../file/2689_0.jpg" /></p></p><hr/><h3>输入格式</h3><p><div style="text-indent: 20.5pt"><span style="font-size: 12pt">第一行有一个正整数</span><span style="font-size: 12pt">N</span><span style="font-size: 12pt">，表示城市数。</span></div>
<div style="text-indent: 20.5pt"><span style="font-size: 12pt">接下来</span><span style="font-size: 12pt">2*N-3</span><span style="font-size: 12pt">行（环上有</span><span style="font-size: 12pt">N</span><span style="font-size: 12pt">条公路，环中有</span><span style="font-size: 12pt">N-3</span><span style="font-size: 12pt">条公路），每行两个正整数</span><span style="font-size: 12pt">u</span><span style="font-size: 12pt">、</span><span style="font-size: 12pt">v</span><span style="font-size: 12pt">，表示城市</span><span style="font-size: 12pt">u</span><span style="font-size: 12pt">与城市</span><span style="font-size: 12pt">v</span><span style="font-size: 12pt">之间有一条公路。</span></div></p><hr/><h3>输出格式</h3><p><div style="text-indent: 20.5pt"><span style="font-size: 12pt">仅有一行一个正整数，表示最少的堡垒数。</span></div></p><hr/><h3>样例输入</h3><pre>8
1 2
2 3
3 4
4 5
5 6
6 7
7 8
8 1
1 3
8 3
7 3
7 5
5 3
</pre><hr/><h3>样例输出</h3><pre>4</pre><hr/><h3>提示</h3><p><p><br />
【样例说明】<br />
在1、3、5、7号城市建堡垒即可。<br />
【数据规模和约定】<br />
30%的数据中：N&lt;=1000。<br />
100%的数据中：N&lt;=100000。</p></p><hr/><h3>题目来源</h3><p>没有写明来源</p>
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